Add support for React 17's automatic JSX transform (#334, #718, #1172, #2318, #2349)

This adds support for the new "automatic" JSX runtime from React 17+ to esbuild for both the build and transform APIs.

New CLI flags and API options:

• --jsx, jsx — Set this to "automatic" to opt in to this new transform
• --jsx-dev, jsxDev — Toggles development mode for the automatic runtime
• --jsx-import-source, jsxImportSource — Overrides the root import for runtime functions (default "react")

New JSX pragma comments:

• [@jsxRuntime](https://github.com/jsxRuntime) — Sets the runtime (automatic or classic)
• [@jsxImportSource](https://github.com/jsxImportSource) — Sets the import source (only valid with automatic runtime)

The existing [@jsxFragment](https://github.com/jsxFragment) and [@jsxFactory](https://github.com/jsxFactory) pragma comments are only valid with "classic" runtime.

TSConfig resolving: Along with accepting the new options directly via CLI or API, option inference from tsconfig.json compiler options was also implemented:

• "jsx": "preserve" or "jsx": "react-native" → Same as --jsx=preserve in esbuild
• "jsx": "react" → Same as --jsx=transform in esbuild (which is the default behavior)
• "jsx": "react-jsx" → Same as --jsx=automatic in esbuild
• "jsx": "react-jsxdev" → Same as --jsx=automatic --jsx-dev in esbuild

It also reads the value of "jsxImportSource" from tsconfig.json if specified.

For react-jsx it's important to note that it doesn't implicitly disable --jsx-dev. This is to support the case where a user sets "react-jsx" in their tsconfig.json but then toggles development mode directly in esbuild.

esbuild vs Babel vs TS vs...

There are a few differences between the various technologies that implement automatic JSX runtimes. The JSX transform in esbuild follows a mix of Babel's and TypeScript's behavior:

• When an element has __source or __self props:

  • Babel: Print an error about a deprecated transform plugin
  • TypeScript: Allow the props
  • swc: Hard crash
  • esbuild: Print an error — Following Babel was chosen for this one because this might help people catch configuration issues where JSX files are being parsed by multiple tools

• Element has an "implicit true" key prop, e.g. <a key />:

  • Babel: Print an error indicating that "key" props require an explicit value
  • TypeScript: Silently omit the "key" prop
  • swc: Hard crash
  • esbuild: Print an error like Babel — This might help catch legitimate programming mistakes

• Element has spread children, e.g. <a>{...children}</a>

  • Babel: Print an error stating that React doesn't support spread children
  • TypeScript: Use static jsx function and pass children as-is, including spread operator
  • swc: same as Babel
  • esbuild: Same as TypeScript

Also note that TypeScript has some bugs regarding JSX development mode and the generation of lineNumber and columnNumber values. Babel's values are accurate though, so esbuild's line and column numbers match Babel. Both numbers are 1-based and columns are counted in terms of UTF-16 code units.

This feature was contributed by @jgoz.